Biological fitness can be measured by the number of offspring an individual produces during his or her lifetime. What biological features increase fitness is defined by the environment which, in human context, includes the society. It is then no wonder that the welfare state has changed the environment so that people who less than a hundred years ago would have been considered "unfit" are now among the fit thanks to taxpayer-funded welfare checks.
One could draw the apparent conclusion that we're on a degenerative path heading towards some kind of an idiocracy. Obviously, this won't happen as the welfare system is dependent on production which in turn is defined by how the society allocates its factors of production; capital and labor. The overall tax burden cannot be increased indefinitely without causing the system to collapse (at high tax rates, government revenue starts to fall) which would leave the high-breeding, non-working individuals without a safety net.